This invention relates generally to a development apparatus for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to a process of loading the surface of an interdigitated electroded donor roll with uncharged toner particles, subsequently corona charging the toner, and forming a toner cloud in a development zone.
Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from either a scanning laser beam, an LED array or an original document being reproduced. By selectively discharging certain areas on the photoconductor, an electrostatic latent image is recorded on the photoconductive surface. This latent image is subsequently developed by charged toner particles supplied by the development sub-system.
Powder development systems normally fall into two classes: two component, in which the developer material is comprised of magnetic carrier granules having toner particles adhering triboelectrically thereto and single component, which typically uses toner only. The development system disclosed herein is of the latter, or single component, type. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface The toner powder image is subsequently transferred to a copy sheet, and finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
The adhesion of charged toner particles in large part determines the operating latitude of powder xerographic development systems. It has been found that triboelectrically charged toner has high electrostatic adhesion, due to non-uniform surface charge distributions and localized regions of high surface charge density on the toner particles. The high adhesion of tribo-charged toner severely restricts the operating latitude of powder development systems, particularly those in which a toner cloud is generated to develop the latent image.
For powder xerography, the image quality requirements make it necessary to reduce the toner particle size to around 5 microns or less in diameter. For printers serving the color offset printing markets, the development system requires high quality, high speed and robust toner delivery. The ability to blend different color toners to achieve custom colors is another requirement. Unfortunately, traditional powder development systems based on triboelectric toner charging do not appear to have the operating latitude necessary to simultaneously satisfy all of these requirements. As will be demonstrated below, however, the use of an ion charging-based development system potentially enables the extended capabilities required for high quality production color printing with dry powder.
The operating latitude of a powder xerographic development system is determined to a great degree by the ease with which toner particles are supplied to an electrostatic image. Placing charge on the particles, to enable movement and imagewise development via electric fields, is most often accomplished with triboelectricity. However, all development systems which use triboelectricity to charge toner, whether they be two component (toner and carrier) or monocomponent (toner only), have one feature in common: charges are distributed non-uniformly on the surface of the toner. This results in high electrostatic adhesion due to locally high surface charge densities on the particles. Toner adhesion, especially in the development step, is a key factor which limits performance by hindering toner release. As the toner particle size is reduced to enable higher image quality, the charge Q on a triboelectrically charged particle, and thus the removal force (F=QE) acting on the particle due to the development electric field E, will drop roughly in proportion to the particle surface area. On the other hand, the electrostatic adhesion forces for tribo-charged toner, which are dominated by charged regions on the particle at or near its points of contact with a surface, do not decrease as rapidly with decreasing size. This so-called "charge patch" effect makes smaller, tribo-charged particles much more difficult to develop and control.
Jumping development systems, in which toner is required to jump a gap to develop the electrostatic latent image, are capable of image quality which can be superior to in-contact systems, such as magnetic brush development. Unfortunately, they are also much more sensitive to toner adhesion. In fact, high toner adhesion has been identified as a major limitation in jumping development. Up to now, mechanical and/or electrical agitation of toner have been used to break these adhesion forces and allow toner to be released into a cloud for jumping development. This approach has had limited success, however. More agitation often releases more toner, but high adhesion due to triboelectric charging still dominates in toner cloud generation and causes unstable development. For full color printing system architectures in which the complete image is formed on the image bearing member, an increase in toner delivery rate produces a highly interactive toner cloud, which disturbs previously developed particles on the latent image. This erases many of the original benefits of jumping development for color xerographic printing for the so-called image-on-image (IOI) architecture. Again, as the toner size is reduced, the above limitations become even more acute due to increased toner adhesion.
Given that charged particle adhesion is a major limiting factor in development with dry powder, it has been a goal to identify toner charging and delivery schemes which keep toner adhesion low. Clearly, the adhesion of the charged toner depends sensitively on the method used to charge the particles. Triboelectric charging is known to produce highly adhering particles. On the other hand, ion toner charging, which occurs when toner particles capture ions emitted by a nearby corona device, results in a more uniform deposition of charge on the particle's surface, and thus lowers the adhesion of the particles for a given charge level.
It is well known that fluidizing reservoirs, commonly referred to as fluidized beds, provide a means for storing, mixing and transporting toner in certain single component development systems. Efficient means for fluidizing toner and charging the particles within the fluidized bed are disclosed in U.S. Pat. No. 4,777,106 and U.S. Pat. No. 5,532,100, which are hereby incorporated by reference. In these disclosures, corona devices are embedded in the fluidized toner for simultaneous toner charging and deposition onto a receiver roll. While the development system as described has been found satisfactory in some development applications, it leaves something to be desired in the way in applications requiring the blending of two or more dry powder toners to achieve custom color development. Also, it has been found in the above systems that there are frequently disturbances to the flow in the fluidized bed associated with charged particles in the high electric fields surrounding corona devices immersed in the reservoir. Finally, it is known that residual toner left on the donor roll after development contributes to non-uniformities in subsequently loaded toner layers, thereby leading to the so-called "ghosting" defect in printed images.
Briefly, the present invention obviates the problems noted above by enabling a gentle toner handling system in which non-contact metering and particle charging on an electroded donor roll can be controlled independently to provide charged toner particles with low adhesion for xerographic development. The toner is initially extracted electrostatically from a fluidized bed and deposited as a net neutral layer on a donor member. This toner layer is subsequently charged with a DC or AC corona device and delivered to a latent image. This so-called ion charging produces a more uniform deposition of charge on the toner particles, resulting in significantly lowered particle adhesion. In addition, the ion charging process is independent of toner pigment, allowing mixtures of two of more different colored toners to be charged homogeneously. Residual toner on the donor is neutralized and returned to the fluidized bed toner reservoir during each complete cycle of the donor roll.
There is also provided an apparatus for developing a latent image recorded on an imaging surface, comprising; a housing defining a reservoir storing a supply of developer material comprising toner; means for fluidizing said developer material in the chamber of said housing; a donor member, mounted partially in said chamber and spaced from the imaging surface, for transporting toner on an outer surface said donor member to a region opposed from the imaging surface, said toner donor member having a plurality of electrodes positioned near the outer surface of donor member; means for electrical biasing a portion of said electrode members on a region of said donor member positioned in close proximity to said fluidized toner so as to electrostatically load toner onto the region of the donor member; means for ion charging said toner loaded on the region of said donor member; means for electrical biasing said electrode members positioned in close proximity to said imaging member to detach toner from said region of said donor member as to form a toner cloud for developing the latent image; and means for discharging and removing residual toner on the region of said donor and returning said toner to the reservoir.